Emma Cathleen Steigerwald, Cassandra Gendron, Juan Carlos Chaparro, Rosemary G. Gillespie, Allison Q. Byrne, Rasmus Nielsen, Erica Bree Rosenblum

The climate-driven range shifts of host species could potentially impact emerging infectious disease (EID) events through several mechanisms, with repercussions for conservation and public health. Host range expansion could affect infection outcomes if hosts and pathogens respond differentially to new environments or create novel transmission opportunities if new contact is established between alternate competent host species or populations. Here, we study Marbled four-eyed frogs (Pleurodema marmoratum) and their fungal pathogen Batrachochytrium dendrobatidis (Bd) in the Cordillera Vilcanota, Peru. There, these frogs were recorded as having expanded hundreds of vertical meters into deglaciating habitats to become the highest-living amphibians globally. With field surveys, we establish that range expansion created new opportunities for Bd transmission: P. marmoratum is now continuously distributed along passages between populations otherwise separated by heavily-glaciated mountains. We sequence Vilcanota Bd, finding that it belongs to the lineage most frequently associated with amphibian declines (BdGPL) and characterizing it as lacking genetic structure despite possessing abundant variation, consistent with extensive Bd dispersal. Collecting temperature data from P. marmoratum microhabitats, we demonstrate that upslope expansion clearly exposed frogs and their pathogen to new thermal regimes. Finally, we analyze field and infection data from individual frogs, concluding that the new elevations colonized by P. marmoratum appear to moderately constrain Bd infection intensities and influence the sublethal costs of infection: infected frogs at high elevation do not have depressed body conditions like their low-elevation counterparts, but also do not achieve the larger body sizes that uninfected individuals typically reach at higher elevations.